Studies of low-energy electron attachment at surfaces

Abstract

Studies of the formation of negative ions at heated metal surfaces are reported under two experimental conditions: (1) laser desorption/ablation ionization and (2) heated metal wires in the presence of fluorine gas. In condition (1), nanoclusters of boron nitride/graphite are first produced by laser ablation of boron nitride mixed with graphite in a heated (∼1100 °C) rare gas followed by laser desorption negative ionization to yield a wide variety of cluster anions. The negative ion mass spectra for laser ablation/desorption ionization of small cluster ions from fullerenes, graphite, or most carbon containing metals (e.g. stainless steel) show common features (C n − for n = 1 to ∼10, with even > odd alternation). The laser serves to heat the surface, produce clusters, and provides free and “quasifree” electrons for attachment and to dissociate larger negative ions/neutrals to produce low-mass cluster anions. Laser desorption of C 60H 36 and C 60F 48 at high laser power results in intense H − and F − ion signals, respectively. In the case of C 60H 36, the H − ion production is attributed to dissociative electron attachment, i.e. e + C 60H 36 → C 60H 35 + H −. For the case of C 60F 48, both dissociative attachment and photodissociation of C 60F 48 − are believed responsible for the F − ion yield. These observations form the basis for the development of intense pulsed ion sources of H − and F − ions for use in energy production (fuel injection into fusion reactors), spallation neutron devices, lithography, and other applications. In condition (2), heated metal wires of Al, Au, Au/Pd, Nb, Ni/Cr, Pt, Re, Ta, Ti, V, W, and Zr are “burned” in a low pressure vapor of fluorine gas (∼10 −4 Torr) resulting in a wide variety of molecular anions (e.g. Al 2F 9 −; AuF 2,3 −; NbF 6 −; ReF 5,6 −; TaF 6 −; Ti 4,5 −; VF 4,5 −; WF 5,6 −; ZrF 5 −, and Zr 2F 9 −) of varying intensity. Also of interest are the occurrences of F 3 − and the “impurity” ion Na 2F 3 −.